Eukaryotic mRNAs bear a “cap” structure at their 5′-termini that is well known to play an important role in translation. Naturally occurring cap structures consist of a 7-methyl guanosine that is linked via a triphosphate bridge to the 5′-end of the first transcribed nucleotide, resulting in m7G(5′)ppp(5′)N, where N is any nucleotide. The mRNA cap plays an important role in gene expression. It protects the mRNAs from degradation by exonucleases, enables transport of RNAs from the nucleus to the cytoplasm, and participates in assembly of the translation initiation complex. m7G(5′)ppp(5′)G (mCAP) has been used as the primer in transcription with T7 or SP6 RNA polymerase in vitro to obtain RNAs having a cap structure in their 5′-termini. In vivo, the cap is added enzymatically. However, over the past 20 years or so, numerous studies have required the synthesis of proteins in an in vitro translation extract supplemented with in vitro synthesized mRNA. The prevailing method for the in vitro synthesis of capped mRNA employs a pre-formed dinucleotide of the form m7G(5′)ppp(5′)G as an initiator of transcription. A disadvantage of using mCAP, a pseudosymmetrical dinucleotide, has always been the propensity of the 3′-OH of either the G or m7G (m7Guo) moiety to serve as the initiating nucleophile for transcriptional elongation. This disadvantage was addressed by provision of modified cap analogs having the 3′-OH group of the m7G portion of the cap blocked to prevent transcription from that position.
In the cell, the cap is added in the nucleus and is catalyzed by the enzyme guanylyl transferase. The addition of the cap to the 5′ terminal end of RNA occurs after transcription but immediately after transcription initiation so that it is almost impossible to detect. The terminal nucleoside is always a guanine, and is in the reverse orientation to all the other nucleotides, i.e., 5′Gppp5′GpNpNp . . . and the cap contains two nucleotides, connected by a 5′-5′ triphosphate linkage.
Transcription of RNA usually starts with a nucleoside triphosphate (usually a purine, A or G). When transcription occurs in vitro, it typically includes a phage RNA polymerase such as T7, T3 or SP6, a DNA template containing a phage polymerase promoter, nucleotides (ATP, GTP, CTP and UTP) and a buffer containing magnesium salt. The 5′ cap structure enhances the translation of mRNA by helping to bind the eukaryotic ribosome and assuring recognition of the proper AUG initiator codon. This function may vary with the translation system and with the specific mRNA being synthesized.
During translation the cap is bound by translation initiation factor eIF4E and the cap-binding complex (CBC) recruits additional initiation factors. Decapping is catalyzed by proteins dcp1 and dcp2 which compete with eIF4E to bind to the cap. Translation results in amino acids as encoded by the mRNA to join together to form a peptide and occurs as three processes, initiation, elongation and termination. Initiation in eukaryotes involves attachment of a ribosome which scans the mRNA for the first methionine codon. Elongation proceeds with the successive addition of amino acids until a stop codon is reached, terminating translation.
Capped RNA encoding specific genes can be transfected into eukaryotic cells or microinjected into cells or embryos to study the effect of translated product in the cell or embryo. If uncapped RNA is used, the RNA in these experiments is rapidly degraded and the yield of translated protein is much reduced.
Capped RNA can also be used to treat disease. Isolated dendritic cells from a patient can be transfected with capped RNA encoding immunogen. The dendritic cells translate the capped RNA into a protein that induces an immune response against this protein. In a small human study, immunotherapy with dendritic cells loaded with CEA capped RNA was shown to be safe and feasible for pancreatic patients (Morse et al., Int. J. Gastroinstest. Cancer, 32, 1-6, (2002)). It was also noted that introducing a single capped RNA species into immature dendritic cells induced a specific T-cell response (Heiser et al., J. Clin. Invest., 109, 409-417 (2002)).
Thus, there is a need for mRNA cap analogs to produce capped mRNA in vitro.